Field of the Invention
The present invention relates generally to measurement systems and more particularly to systems and methods for improving the performance and reducing the cost of Capacitance Diaphragm Gauges (xe2x80x9cCDGsxe2x80x9d) which utilize dual electrode Capacitance Diaphragm Sensors (xe2x80x9cCDSsxe2x80x9d) by improving the electrical interface to the CDS.
Background of the Invention
Capacitance diaphragm gauges (or capacitance diaphragm manometers) are widely used in the semiconductor industry. In part, this is because they are typically well suited to the corrosive services of this industry. They are also favored because of their high accuracy and resistance to contamination. In particular, those CDGs in which the CDS is heated exhibit enhanced resistance to contamination and operate longer without maintenance.
A CDS serves as the vacuum/pressure sensing element within a CDG and may be used to measure and/or control the pressure within a process chamber. A CDS has a housing containing two chambers separated by a circular tensioned diaphragm. The first chamber is in fluid communication with the process chamber or other assembly in which the pressure is to be measured. The second chamber of the CDS is commonly referred to as the reference chamber and is typically (although not necessarily) evacuated and sealed at a pressure which is substantially less than the minimum pressure the sensor will be required to resolve.
The circular, tensioned diaphragm (xe2x80x9cthe diaphragmxe2x80x9d) which separates the two chambers within a CDS housing is essentially a thin metal diaphragm which is mechanically constrained about its periphery. The diaphragm reacts to differential pressures by deforming into a bowed shape with the periphery remaining stationary. The diaphragm thereby serves as a flexing, grounded electrode. The diaphragm deforms as a reaction to the pressure difference across it and also interacts with electrostatic fields such that the deformation of the diaphragm may be resolved through these electrostatic interactions.
In close proximity to the diaphragm lies the electrode assembly. This assembly consists of a stiff platform with a polished, electrically insulating surface, which bears two conductive electrodes. The electrode assembly is mechanically constrained a fixed distance from the plane containing the periphery of the diaphragm so that the electrodes are very close to the diaphragm ( less than 0.005 in) and run parallel to its surface. Flexure of the diaphragm, due to applied pressure, can easily be computed by measuring the capacitance to ground at each electrode and subtracting one measurement from another.
Modern CDSs utilize two electrodes to monitor the flexure of the diaphragm. The capacitance to ground of the two electrodes (xe2x80x9ccommon-mode capacitancexe2x80x9d) varies with flexure of the diaphragm, but also changes with movement of the electrode assembly. Such movement occurs with temperature changes, temperature transients, and mechanical loading. Measurements using the difference in capacitance of the two plates (xe2x80x9cdifference capacitancexe2x80x9d) are more stable since they reject motions between the diaphragm and electrodes and instead reflect the deflection of the diaphragm.
Systems that utilize CDGs generally have stringent requirements for the repeatability of pressure readings, with offset drift typically limited to 0.02% of full scale per day. Full-scale deflection typically results in differential capacitance of 0.2 2.0 pF(10xe2x88x9212 F). 0.02% of this value gives an allowable equivalent change of 0.04-0.4 femtoFarad (10xe2x88x9215 F) per day, where some of the change is due to electrical errors when measuring and subtracting the capacitance at the CDS.
The measurement of the CDSs capacitances is performed by the Analog Front End (xe2x80x9cAFExe2x80x9d) electronics. The AFE is not only responsible for interfacing to the CDS, it also performs the subtraction operation which gives the difference capacitance. Since the full-scale difference capacitance may be as low as 0.2 pF (10{circumflex over ( )}xe2x88x9212 F) with a common mode capacitance of 68 pF, even at full-scale, the common mode capacitance is 340 times greater than the difference we wish to measure. For a case involving a daily allowable drift of 0.04 femtoFarad, the common mode capacitance is about 1.7 million times the allowable variation in difference capacitance. Thus the subtraction operation must be extremely well balanced and stable to ensure that the AFE maintains a reasonable drift error.
An obvious source of measurement error within a CDG is the accumulation of incidental capacitance due to interactions between circuits, lead wires, and structures within the construction. These effects even occur on the circuit board and within integrated circuits bringing along leakage currents which makes the circuitry sensitive to humidity and contamination. The solution to these leakage elements is guarding. A node which is surrounded by a conducted surface bearing the same voltage (a xe2x80x9cguarding surfacexe2x80x9d) generally will not experience capacitance or leakage current. By surrounding important nodes with guarding, they are free to operate without interference and variations due to shifting, flexing, or changes in humidity. The source and greatest need for guard potential lies, for the most part, in the AFE.
Given the stringent performance requirements of the AFE, few circuit topologies have proven suitable. Three topologies currently dominate the CDG market: the balanced diode bridge; the guarded-secondary transformer bridge; and the matched reference-capacitor bridge.
The balanced diode bridge topology, which is illustrated in FIG. 1, utilizes an excitation source, which provides an alternating voltage to drive the electrodes of the CDS. Charge is alternately supplied to and removed from each of the electrodes through a diode bridge to and from capacitors, CA and CB. Each of the capacitors serves to supply current to one electrode while discharging current from the other. Thus any imbalance in the capacitance to ground of the two electrodes results in a voltage difference between the output pins of CA and CB.
Diode bridge AFEs are simple and inexpensive, which makes them a suitable choice for less demanding applications, such as 10 Torr unheated sensors. With stabilized temperature and humidity, they have even been used down to 100 mTorr. However, they generally need to be in close proximity to the CDS since they lack an easy means of producing a useful guard potential. Also, they suffer from diode mismatch and boar contamination issues.
Referring to FIG. 2, a guarded-secondary transformer-based bridge is shown. This circuit utilizes a center-tapped secondary constructed of coaxial cable to produce the excitation voltage along with proportionally increasing guard voltage. The current induced in the CDS by the excitation voltage flows from one electrode of the sensor to the other. Thus, charge is conserved and any differential capacitance results in a net voltage at the center-tap of the innermost conductor of the secondary. A high-input-impedance, unity-gain amplifier follows the center tap of the inner conductor and places a similar voltage on the shield, allowing for guarding. The output of the unity gain amplifier represents the difference capacitance in the sensor. It is amplified and is sent to a synchronous detector to generate a DC level proportional to the difference in capacitance.
When implemented well, the guarded-secondary transformer-based bridge represents a vast improvement over the balanced diode bridge in stability and accuracy. It allows the CDS to be remotely placed through utilization of the same guarded, coaxial cables, which are wrapped about the transformers core. The principal problems with this technology lie in its implementation. The coaxial cable must be of exceptional consistency and must be free of cracks or holes in the shielding. In addition, the guarding method is somewhat Imperfect, being based upon a less than unity gain follower, and stable construction is essential to achieve a stable CDG. Finally, the all shielded construction utilizing coaxial cables can complicate interconnects to and trimming of the CDS.
Referring to FIG. 3, a matched reference-capacitor bridge is shown. This circuit uses a common excitation source to drive both electrodes of the CDS through the summing nodes of a pair of fully-guarded charge amplifiers (guard voltage is on power supplies and common potentials). The charge amplifiers are built around two monolithic operational amplifiers and utilize a pair of matched, precision reference capacitors to establish their gain. The output of the charge amplifiers is fed into a high common-mode rejection difference amplifier. The output of the difference amplifier represents the difference capacitance in the sensor and is sent to a synchronous detector to recover a DC level corresponding to the diaphragm""s position.
The matched reference capacitor bridge competes well with the guarded-secondary transformer-based bridge with the added advantage of being smaller. It is somewhat temperature sensitive, however, and it is extremely demanding with respect to component performance. Only the highest quality glass capacitors are suitable as references, and they must be subjected to aging and matching. Also, the difference amplifier must exhibit outstanding and consistent common-mode performance. With such high performance components, the material cost is high for this topology.
A need exists for an AFE topology which utilizes standard construction practices and inexpensive components while being relatively insensitive to temperature and humidity. This topology should feature excellent guarding to allow for ease of connection to a separate, heated sensor assembly and should allow for direct compensation of the sensor""s offset.
One or more of the problems outlined above may be solved by the various embodiments of the invention. Broadly speaking, the invention comprises systems and methods for determining differential currents in a pair of reactive circuits. In a preferred embodiment, the systems and methods are implemented to provide an improved electrical interface to a sensing element such as a CDS. The improved interface may provide improved performance and reduced cost.
One embodiment of the invention comprises an interface for a CDS utilizing a differencing current transformer and a charge amplifier. The primary windings of the current transformer are coupled between an excitation voltage source and a pair of electrodes within the CDS. The currents passing through the primary windings generate magnetomotive force (MMF) of opposing polarity, so that a current proportional to the difference between the primary currents is induced in the secondary winding of the transformer. The secondary winding is coupled to the summing node of a charge amplifier, thus terminating the secondary of the transformer into a low impedance load. This low impedance termination is reflected back to the primary windings and appears as smaller impedance than the CDS""s electrodes. Thus, the voltage presented to the electrodes is extremely close to the excitation voltage, and the excitation voltage becomes an excellent guard potential. Every circuit node associated with the transformer and charge amplifier are referenced, AC-wise, to the guard potential and shielded. This protects the transformer, charge amplifier, and any interconnects from stray capacitance and leakage currents.
Embodiments of the invention typically (though not necessarily) operate the current transformer and its related components at excitation voltage for reasons of guarding as well as providing excitation to the electrodes of a grounded CDS. One embodiment of the invention utilizes a common-mode transformer to translate power supply potentials, as well as signal from the charge amplifier, between those circuits referenced to the excitation voltage and those circuits referenced to ground. An additional winding on the transformer is excited by the excitation voltage and ground, and serves as a source for the other signals, which must be translated. There exist other embodiments of the invention which essentially perform the same operation without the transformer.
The voltage signal recovered from the charge amplifier passes through the common-mode transformer and is fed into a synchronous detector which serves to generate a DC level corresponding to the CDS""s applied pressure. The actual means of delivering the signal from the charge amp to the detector may vary, particularly in regard to gain stages.
Another embodiment of the invention comprises a method for measuring current differential comprising coupling one or more primary windings of a current transformer to each of a pair of circuits in which a current differential is to be measured, applying a excitation voltage to the primary windings and corresponding circuits, inducing a differential current on a secondary winding of the current transformer and amplifying the current differential using a charge amplifier. The method may further comprise referencing the charge amplifier to the excitation voltage and guarding the current transformer and charge amplifier by surrounding them with a shielding structure to which the excitation voltage is also applied. The excitation voltage which is added to the charge amplifier signal can then be removed by passing the signal through a common mode transformer on one winding, while the excitation voltage is applied to another winding. The resulting signal can then be fed to a synchronous detector, which produces a DC level output indication of the current differential.
Numerous alternative embodiments are also possible.